U.S. Pat. No. 8,652,383 discloses an insert part weld molding method for reliably welding an insert part to a blow molded object while preventing an unwelded portion from being created at a circumferential edge of the insert part in a thickness direction thereof. The insert part weld molding method welds a blow molded object to an insert part 3 set in a mold 7, wherein a molding surface 7a of the mold 7 is set higher than a front surface 6d of a portion to be welded of the insert part 3, and an entire surface of a circumferential edge 6b of the insert part 3 in the thickness direction thereof and a parison 11 of the blow molded object are brought into a gapless welded state. Also adopted is a method of suppressing a decrease in temperature of the insert part 3 by arranging a preheating plate 8 which is made of a high thermal conducting material and which is to become a part of the molding surface 7a of the mold 7 in a vicinity of a portion where the parison 11 and the circumferential edge 6b of the insert part 3 in the thickness direction thereof are welded, and arranging a heat insulating plate 9 between the preheating plate 8 and the mold 7.